Foraminous electrostatographic transfer system

ABSTRACT

In electrostatographic apparatus for applying a high bias voltage between a roller electrode and a support surface to provide an electrical field for development material transfer between them without arcing or undesired corona, a roller electrode having an electrically conductive core to which said high voltage is applied and a normally thick roller body of foraminous open cell material highly compressed between the conductive core and the support surface.

United States Patent Gundlach 1 Feb. 18, 1975 FORAMINOUSELECTROSTATOGRAPHIC 3,633,543 1/1972 Pitasi 118/621 E 3,744,896 7/1973Carreira 355/3 TRANSFER SYST M 3,776,723 12/1973 Royka ct a1 [75]Inventor: Robert W. Gundlac V NY. 3,796,183 3/1974 Thettu 118/70 73 Anee: Xerox Cor oration Stamford, 1 851g Conn p Primary Examiner-MervinStein Assistant Examiner-Leo Millstein [22] Filed: May 29, 1973 [21]Appl. No.: 364,463 [57] ABSTRACT In electrostatographic apparatus forapplying a high 52 us. 01 118/637, 96/1.4, 117/175, bias voltage betweena roller electrode and a pp 55 surface to provide an electrical fieldfor development [51] Int. Cl G03g 13/00 material transfer between themWithout arcing 0F 58 Field of Search 118/637; 117/175, 93.4; desiredCorona, a roller electrode having an electri- 96/14; 55 cally conductivecore to which said high voltage is ap plied and a normally thick rollerbody of foraminous [56] References Cited open cell material highlycompressed between the UNITED STATES PATENTS conductive core and thesupport surface.

3,626,260 12/1971 Kimura et a1. 317/262 18 Claims, 3 Drawing Figures f-n 5/ 4- 54 u X/II PATENTEDFEBI8I9Y5 3,866,572 SHEET 1 OF 2 FIGJ FIG. 2m

FORAMINOUS ELECTROSTATOGRAPHIC TRANSFER SYSTEM The present inventionrelates to the transfer of image developing charges or materials fromone support surface to another in electrostatography, and moreparticularly to the use of endless foraminous members in connection withelectrical fields for such transfers.

The best known example of such transfer is the conventional transferstep in xerography wherein toner is transferred from the photoreceptor(the original support surface) to the copy paper (the final surface).However, such development material transfers are required in otherelectrostatographic processing steps, such as electrophoreticdevelopment. In xerography, developer transfer is most commonly achievedby electrostatic force fields created by D.C. charges applied to theback of the copy paper (opposite from the side contacting thetoner-bearing photoreceptor) sufficient to overcome the charges holdingthe toner to the photoreceptor and to attract most of the toner totransfer onto the paper. These xerographic transfer fields are generallyprovided in one of two ways, by ion emission from a transfer corotrononto the paper, as in U.S. Pat. No. 2,807,233, or by a D.C. biasedtransfer roller or belt rolling along the back of the paper. Examples ofbias roller transfer systems are described in U.S. Pat. Nos. 2,807,233;3,043,684; 3,267,840; 3,598,580; 3,625,146; 3,630,591; 3,691,993;3,702,482; and 3,684,364. Also, French Pat. No. 2,065,390, Germanapplication OLS No. 2,102,634 and Brtish Pat. Nos. 1,210,666 and1,302,922. U.S. Pat. No. 2,968,555 issued Jan. 17, 1961 to R. W.Gundlach in FIG. 3 and Column briefly discloses a xerographic transfersystem utilizing a soft resilient sponge rubber coated transfer roller,preferably electrically conductive. However, no substantial deformationof this roller is shown or suggested.

Foraminous members have, of course, been utilized in other differentapplications in electrostatography, such as paper handling. Also, aplanar, evenly compressed conductive pressure pad for intermittentlatent (charge) image transfer is disclosed in U.S. Pat. No. 3,635,556issued Jan. 18, 1972, to R. L. Levy.

The transfer of the development materials involves the difficult andcritical physical detachment and transfer over of such particulatematerials by high intensity electrostatic force fields from one surfaceinto attachment with another surface, maintaining the same pattern andintensity as the original latent electrostatic image being reproducedwithout scattering or smearing of the developer material. This difficultrequirement can only be met by careful control of the electrostaticfields, which must be high enough for transfer yet not cause arcing orexcessive corona generation at undesired locations, since suchelectrical disturbances can easily cause scattering or smearing of thedevelopment materials.

It will be noted that in electrophoretic or photoelectrophoreticdevelopment that similar critical problems and requirements are presentfor sensitization and transfer of the image developer material. Ofcourse, the function is somewhat different than for xerographic transfersince the image development materials (originally in a liquidsuspension) are being selectively transferred from the conductive chargeinjecting surface to a blocking electrode surface, which may be thefinal support surface. Conventio'nally, however, the transfer to a finalsupport surface such as paper or plastic sheets or webs of the imageformed on the conductive surface is made in a subsequent transfer step.The electrically biased roller or web electrode in photoelectrophoreticimaging is conventionally called a blocking or imaging electrode ratherthan a bias transfer roll as in xerography. Further, there are importantdistinctions in the desired presence or absence of nip coronageneration. The following U.S. patents are illustrative ofelectrophoretic systems and electrodes and various means by whichcontrol of their arcing or corona has been attempted: U.S. Pat. Nos.3,384,565; 3,474,019; 3,551,320; 3,582,205 and 3,697,407. Of these, U.S.Pat. Nos. 3,474,019 and 3,551,320 illustrate deformable soft electrodes.Canadian Pat. No. 876,045 discloses a PEP blocking electrode with anopen cell sponge-like layer for applying the liquid imaging materialtherefrom.

The critical function of pre and post-nip corona in xerographic biasroll transfer is discussed, for example, in copending application Ser.No. 309,562 filed Nov. 22, 1972, by Thomas Meagher, entitled ConstantCurrent Biasing Transfer System now U.S. Pat. No. 3,781,105, issued Dec.25, 1973, and in U.S. Pat. No. 3,702,482 by C. Dolcimascolo et al.,issued Nov. 7, 1972.

Perhaps the most difficult technical problem in all roll or beltelectrode systems for transferring of electrostatographic imagingdevelopment materials or charges between supports is that of controllingor suppressing arcing and undesired corona generations. In practicalsystems the transfer of materials must be effected while the twosurfaces between which the material is being transferred are both movingat the same speed and in relatively close contact." This as a practicalmatter requires the material transferring electrode to be an effectivelyendless surface of a cylindrical roller or small endless belt. This inturn means that the surface of the roller or belt electrode mustcontinuously move in and out of contact with the original supportsurface. This creates varying width air gaps at each side of the actualcontact area (the nip region). The upstream or entrance air gap isconventionally referred to as the prenip region and the downstream airgap is the post-nip region. Due to the fact that the breakdown voltageacross an air gap is very non-linear with changes in the gap dimensions(this characteristic is known as the Paschen curve) control of arcing orionization in such air gaps when there is a high biasing voltage on theelectrode is very difficult. The higher the applied bias voltage themore difficult such control becomes, yet in many applications highvoltages are either required or highly desirable for efficienttransferring of the material from the original surface or liquidsuspension to the second surface. Further, since the field intensity formaterial transfer is a function of the spacing as well as the appliedpotential the biasing voltage charge is desirably applied as closely aspossible to the original support, again further increasing thedifficulty of preventing voltage breakdown by arcing or excessive coronageneration in the nip itself as well as the pre and postnip gaps.Further, both vector direction and intensity of the applied electricalfields varies at different locations and times relative to the rollerelectrode because the electrical fields are geometrically dependent uponthe electrode configurations, and change as the electrode moves. Thepresent invention provides a system in which such desired high biasingvoltages and close spacings may be maintained with a simple andeffective arrangement also providing desired suppression of arcing andsuppression or control of corona emissions in all of the nip, pre-nipand post-nip regions, for more eflicient and reliable application ofhigh fields for transfer of development materials and other operations.

Discussing in further detail the xerographic bias roller transferprocess, the paper contact with the photoreceptor must precede theapplied build up of high electrostatic fields by the transfer roller fortwo reasons. First, if excessive fields exist when the paper is stillapproaching the toner image on the photoreceptor, then toner particlescan prematurely transfer, spreading as they jump the pre-nip gap,resulting in fuzzy images. Secondly, air ionization in the pre-nip gapcan occur, reversing the polarity of toner particle charges andtherefore preventing their subsequent transfer in the nip. The lattereffect usually occurs intermittently, because it is self-quenching, andso manifests itself in what has been called zebra-stripe transfer. Inthe nip an electrostatic field of about volts per micron is sufficientto transfer loose charged toner particles from the photoreceptor to thepaper surface. However, in order to establish a stable electrostaticbond between the toner and the paper after it is transferred, a netcharge should be applied to the back of the paper 0pposite from thetoner charge sufficient to tack the transferred toner to the paper sothat it will not be dislodged in the subsequent paper handling, whichincludes the stripping of the paper from the photoreceptor. As disclosedin the Thomas Meagher U.S. Pat. No. 3,781,105 Dolcimascolo et al. U.S.Pat. No. 3,702,482, this tacking charge may be created by deliberatelyinducing, but controlling, corona generation in the postnip gap with atransfer roller of an electrically relaxable material. A constantcurrent bias voltage supply can compensate for resistance charges in therelaxable material. Alternatively, if the roller and paper are conductive enough, this tacking charge may be applied to the paper in the nipby the contact between the roller and the paper. It will be noted thatpost-nip corona generation to generate toner tacking charges is notrequired for conventional xerographic corona transfer systems, whereboth transfer and tacking are effected by ionically depositing chargeson the back of the paper.

The transfer of development material disclosed herein may take place inconjunction with, or subsequent to, the application of light to reducethe electrostatic forces retaining the toner on the original supportsurface, although this is not essential. It will also be appreciatedthat the system of the invention may be operated in conjunction withvarious subsequent means for the separation of the second support fromthe first support where desired. That is, various conventional sheetstripping devices and/or electrostatic detacking may be utilized.Further, various different surface configurations of the originalsupport surface may be accommodated.

Exemplary embodiments of the present invention are shown and describedhereinbelow as incorporated in otherwise conventional exemplaryelectrostatographic apparatus and processes. Accordingly, said processesand apparatus need not be described in detail herein, since theabove-cited and other references teach details of various suitableexemplary structures, materials and functions to those skilled in theart. Further examples are disclosed in the books Electrophotography byR. M. Schaffert, and Xerography and Related Processes by John H.Dessauer and Harold E. Clark, both first published in 1965 by FocalPress Ltd., London, England. All of the references cited herein arehereby incorporated by reference in this specification.

Further objects, features and advantages of the present inventionpertain to the particular apparatus, steps and details whereby theabove-mentioned aspects of the invention are attained. Accordingly, theinvention will be better understood by reference to the followingdescription and to the drawings forming a part thereof, which aresubstantially to scale, except as noted. wherein:

FIG. I is a schematic plan view of an exemplary photoelectrophoreticimaging system embodiment in accordance with the present invention;

FIG. 2 is the system and view of FIG. 1, showing the system inoperation; and

FIG. 3 is another exemplary embodiment of the invention, in the form ofa plan view, partially in crosssection, of a xerographic bias rollertransfer system in accordance with the present invention.

Referring to the FIGS. 1-3, it may be seen that FIGS. 1 and 2 illustratea foraminous bias roller system of the invention cooperatively improvingan otherwise conventional photoelectrophoretic system 40 forelectrophoretic development. Since the conventional details thereof arefully described in the above-cited references on electrophoresis, thesedetails need not be described herein. FIG. 3 illustrates a xerographictransfer station incorporating a foraminous bias transfer roller inaccordance with the invention. Here, also, other details of the transfersystem 50 known in the art are taught in the above-incorporatedreferences on bias roller transfer systems and need not be described indetail herein.

Considering first in general both the roller electrodes 42 and 15 of thesystem 40 and 50, respectively, it may be seen from the partialcross-sectioning of these axial plan views that both rollers illustratedhere are nor- .mally cylindrical and the vast majority of theircrosssectional areas comprises a roller body of foraminous open cellmaterial uniformly coaxially surrounding a much smaller central core ofconductive material, such as a solid metal roller. The foraminousmaterial here is shown as cylindrical and continuously bonded to thecentral core surface. However, it will be appreciated that the termsroller electrode or roller as used herein are not intended to be limitedto integral cylindrical rollers. They are also intended to read broadlyon equivalent structures such as moving endless belts of the sameforaminous materials with either roller or stationary (sliding contact)arcuate conductive backing members. Examples of such equivalentstructures are disclosed in the above-cited references.

The foraminous material of the rollers 42 and 15 is, for importantreasons to be discussed subsequently, preferably open cell materialrather than closed cell, i.e., having voids or pores which have openingsto allow expulsion or transfer of the contents of individual cells whenthe cells are compressed. This material is preferably highly foraminous,i.e., the principle volume of the material in its normal uncompressedstate comprises a multiplicity of convolute andseparated random voids orpores closely interspaced throughout the material,

so that the solid material itself can be considered primarily asdiscontinuous cell walls separating these voids and occupying only aminor portion of the total volume of the foamed material. Example ofsuitable materials are 25 pores or 45 pore (45 cells per inch) opencelled polyurethane foam, which may be commercially obtained, forexample, from the Scott Paper Company. The present invention isapplicable to many different foraminous materials, many of which arecommercially available, and given the criteria and teachings herein suchmaterials may be readily selected by one of ordinary skill in the art offoraminous materials.

In the system 40 of FIGS. 1-2, the roller electrode 42 is shown with itsforaminous body 44 normally uncompressed in FIG. 1 and operativelycompressed in FIG. 2, It is compressively rotated by its conductive core43 against an image support surface, here comprising a surface layer 45of liquid electrophoretic developer material on a substrate 46, shownbeing conventionally optically image discharged. A high voltage biassupply 47 is connected between the substrate 46 and the conductive core43 for continuous transfer of development material from the surfacelayer 44 onto the outer surface of the roller 42.

It may be seen that the foraminous body 44 of the roller 42 is highlycompressed from its normal uncompressed radius 48 into close to theradius 49 of the conductive core 43. The maximum compression of theroller body 44, due to the curvature of the core 43 (which is notcompressed), occurs at the normal nip area 51, and the compression issubstantially less in the pre-nip area 52 and post-nip area 53. However,the low durometer resiliency of the foam roller body 44 provides a largearea of roller surface contact 54 with the surface 45, extending wellinto the pre-nip and post-nip areas 52 and 53, covering an area muchlarger than just the normal nip contact area 51.

The minimum distance 56 between the core 43 and the surface 45 is here(and also for roller less that. half the normal uncompressed thicknessof the foraminous material, so that a substantial number of the normallyopen cells in the nip are closed by compression. In many casessubstantially full compression is desired (closing of substantially allcells in the center of the nip). With suitably foraminous materials thespacing may be compressed to approximately only percent of theuncompressed spacing without excessive compression forces. With 50percent compression (not practicably achieveable with a solid roller)the field in the nip can be twice the field in the pre and post-nip airgaps. As may be seen, the allowable deformation is such that the rollermay act more like a rolling pillow or bag on the surface 45 than anormal solid roller. As shown, sufficient lateral roll bulging and deepchordal positioning can be achieved such that the surface contact area54 is substantially as wide as the entire normal roll diameter. However,an uncompressed foraminous layer of only one/fourth to one/half inch inthickness has been successfully utilized.

In comparison, even very soft rubber rollers which are solid cannotachieve the desired forms and close nip spacings of the disclosedforaminous roller electrodes. Solid rollers can only be somewhatdeformed, rather than compressed, causing severe internal stresses onthe material in an attempt to bring the roller core close to the supportsurface. Further, with a foam roller a much lower effective durometercan be achieved than with a solid roller without having to go to amaterial which is so soft as to have poor strength and wear resistanceproperties. The much greater roller surface deformation which can beachieved with relatively light compression pressures in foam rollersprovides a much greater surface contact areas for improved developmentor transfer and/or paper holddown, with much more even and reducedpressures for reduced wear and reduced distortion of components, inaddition to the significant electrical advantages disclosed.

A seal coating 57 may be provided on the circumferential exteriorsurface of the roller electrode 42. The seal coating 42 is liquidimpervious in the surface contact area to prevent the entrance ofexternal liquid into the cells of the foraminous material 44, and toretain any liquid therein, yet is sufficiently elastic and resilientlyconformable to the compression of the foraminous material. An example is10 mil polyurethane. A seal coating of non-compressable material can beused to permit surface speed synchronization of the system.

Considering now the exemplary xerographic transfer system 50 to FIG. 3,the similar bias roller 15 thereof has a foraminous roller body 21 witha conductive core 22 connected to a transfer bias voltage source 23. Thebias source 23 is connected through common grounds 13 to a conductivebacking roll 12 against which the roller 15 is highly compressed withmoderate pressure. Through the nip 17 between the two rollers passes aflexible belt photoreceptor 11 and paper or other final support sheet16. Negatively charged toner particles 10 previously developed onto thephotoconductor l1 surface are retained thereon by positive latent imagecharges 14 until transfer is effected. Here, transfer by the bias source23 occurs from the high fields in the nip area 17 created by theforaminous roller body 21 being highly compressed so that the conductivecore 22 is closely spaced in the nip from the photoconductor.

The outer surface 24 of the bias roller 15 may be that of a seal coat 20corresponding to the seal coat 57 of FIGS. 1 and 2. However, this is notrequired if the foraminous material is not liquid filled, or does notexternally contact liquid inks, etc. The seal coating material can bethe same, or different from, the foraminous material, and may beprovided for wear resistance and cleanability properties in lieu of, orin addition to, liquid sealing.

As discussed in the introduction here, and in the above-citedreferences, particularly the pending application Ser. No. 309,562 byThomas Meagher, in the transfer system 50 it is important to suppresscorona in the pre-nip gap W, while in contrast the generation of acontrolled corona in the post-nip gap Y is desired in order to apply apositive tacking charge 30 on the paper 16 to retain the transferredtoner 10 on the paper when the paper 16 is stripped from thephotoreceptor 11 (forming the gap Z th'erebetween).

The disclosed structure provides greatly improved control capabilitiesas compared to an ordinary transfer roller which maintains a generallycylindrical configuration and simple pre and post-nip air gapconfigurations. Not only is the shape and contact area of the nipdifferent here, but also the foraminous material fills the space betweenthe conductive core and the contacting supportsurface with amultiplicity of small discontinuities provided by the cells in thematerial, thereby providing a greatly improved air ionization controlbarrier. The foraminous material is significantly less compressed (lessdense) in the pre-nip and post-nip areas than in the nip area. That is,it has a much greater thickness and porosity between the conductive coreand the support surface in the normal pre-nip and postnip areas. Theforaminous material extends much further laterally into the pre-nip andpost-nip areas, due to its greater deformation, i.e., it has a muchlarger surface contact area. This increasing thickness and porosity ofthe foraminous material can be utilized to provide a varying ionizationcontrol barrier in the pre-nip and post-nip areas. Further, aspreviously noted, even further ionization control variability betweenthe nip area and the pre and post-nip areas can be provided by' thedegree of compression of the foraminous material in the nip. As thecompression is increased, the individual cells may be collapsed, wherebythe tops and bottoms of the cell walls contact one another. As morecells are collapsed the insulating pockets of air are eliminated and theelectrical properties of the roller in the nip can change dramaticallyfrom those of the normal uncompressed porous material to those of athinner solid roller in the same material. This greatly adds to theeffect of the increase in field strength due to the closer conductorspacing geometry of the nip.

The motor 31 in FIG. 3 driving the roller 15 illustrates a means foreccentrically distorting the configuration of the foraminous material21. A driving torque is applied by this motor 31 or any other supportsurface against which the roller is being deformed, causing theforaminous material to bulge out somewhat further in the pre-nip areaand to be withdrawn somewhat in the post-nip area. Thus, both the areaof surface contact and the thickness of the foraminous material in theprenip area may be made substantially greater than that in the post niparea. This provides a greater corona control barrier of such foraminousmaterial in the pre-nip region than the post-nip region, which can beused to assist in providing a desired (above-discussed) suppres sion ofpre-nip gap corona while simultaneously inducing post-nip gap corona,with the same bias level, as

shown.

It will be noted that the deformed radius of curvature of the roller 15or 42 surface at the post-nip exit is much less than the normal rollradius. This sharp curvature, together with the paper beam strength,assists in assuring stripping of the paper from the transfer roll.

The foraminous bias roller systems disclosed herein may be utilized inat least three different material modes, with different operationalproperties andfunctions, although the above-described features andadvantages are applicable to all three. One mode is to provide aforaminous material which is highly insulative, and thereforenon-conductive to the bias voltage supply. Another mode is to provide aforaminous material which is resistive, but at least semi-conductive,such as an electrically relaxable material as disclosed in theabove-cited references on bias transfer rollers. In this second mode atleast part of the transfer bias charge will be conducted out toward theouter surface of the transfer roller, especially during compression. Ina third mode, the foraminous material may be liquid filled. That is, theopen pores of the material may be filled with a selected liquid. Thislatter mode has advantages in better controlling the resistivity of theforaminous material, since in the case of a liquid filled material thecell pores are not being exposed to humidity changes of ambient air asthey open after the compression in the nip. As will-be subsequentlynoted, liquids of a formulated constant resistivity may be utilized toprovide a selected bias charge transfer throughthe foraminous materialby means of the liquids electrical conductivity.

Considering first the mode wherein the foraminous roller body isinsulative and the pores are air filled, it may be seen that in thiscase the foraminous material does not affect the vector direction orintensity of the transfer fields. These fields will be controlledentirely by the geometry of the spacing between the conductive core andthe conductive support surface. In this case the above-describedfunction of the foraminous material in breaking up the air gap into manysmaller individual air gaps separated by cell walls provides animportant function. The foraminous material allows the application ofbiasing potentials of over 1,000 volts between the conductive core andthe support surface with very close nip proximity therebetween toprovide very high field intensities. The foraminous material can allowsuch high field intensities while either totally suppressing ionizationin the entire air gap, or allowing some ionization in post-nip andsuppressing it in the pre-nip and nip areas. However, in the case ofeither highly insulative or highly conductive foraminous materials itwill be appreciated that the fields in the prenip and post-nip gaps willbe symmetric for any applied charge, and that therefore it is notpossible to simultaneously induce corona in one but not the other.

As will be recalled, the electrical field intensity at which any airbreakdown occurs is a function of the air gap distance, and a smallergap will support a much higher field intensity without breakdown. Thisis represented by the characteristic Paschen curve. The wide lateralextent of the roller contact area and its relatively even pressureinsures that the air gap between the outer surface of the roller and thesupport surface is small and substantially constant to well outside ofthe normal nip areas, thereby suppressing arcing or undesired corona. Nolarger air gaps which would induce ionization are formed until thedistance from the conductive core is so great that the field intensityor stress in the air gaps is below the ionization potential, i.e., thefield intensity is greatly reduced by the time the larger pre orpost-nip air gap is formed. With the foraminous roller, the normalposition of the pre-nip and post-nip gaps can be greatly laterallydisplaced, yet simultaneously the distance between the conductorsforming the transfer field can be made very small. These twointer-related desired criteria cannot be effectively met by a solidroller. They can be readily met by a foraminous roller body which issufficiently thick and sufficiently compressible.

Considering now the materials mode in which the foraminous material ofthe bias roll is resistive rather than insulative, as noted above, thismode can be used to provide unsymmetrical fields. That is, the internalresistivity relaxation properties of the material can providesuppression of ionization in the pre-nip air gap while simultaneouslyencouraging it in the post-nip air gap. Suitable material resistivitiesfor such relaxable transfer operations are discussed in above-citedreferences such as the Dolcimascolo, et al., U.S. Pat. No. 3,702,482 andthe Thomas Meagher application. There a nonsymmetrical fielddistribution arises from relaxation of fields within the resistiveroller over time. Here we have the additional advantage of changes inthe effective bulk resistivity itself.

The foraminous material of the invention can provide significantimprovements in such systems because the bulk resistivity (resistanceper unit volume) of the foraminous material can be changed substantiallyby its compression. Thatis, as the foraminous material is compressed inthe nip the actual resistance between the conductive core and the rollersurface decreases. Therefore, the roller resistance and relaxation timein the nip is greatly lower than in the uncompressed prenip area of theroller. This allows a faster relaxation of the material in the nip,which in turn provides higher fields between the roller surface and thesupport surface in the nip and in post-nip over a much greater area.Accordingly, the effective latitudes for transfer are much greaterelectrically as well as mechanically. This allows a relatively higherresistivity material to be used, which material can be less humiditysensitive as to its resistivity and, therefore, more reliable. Suchhigher resistivity material, where foraminous, can insure pre-nip coronasuppression even with substantial variations in humidity and paper, yetalso prevent inadequate relaxation (excessive roll internal fields)which could cause inadequate transfer fields or inadequate post-nip gapionization fields. In fact, the foraminous relaxable material caneffectively act as a complete insulator on the uncompressed entranceside of the nip.

Resistivity changes under compression for foraminous relaxable materialsof several orders of magnitude have been experimentally observed, whichclearly allows greatly improved design and operating control over chargerelaxations. That is, once the tops and bot toms of the cell walls touchone another in compression the resistivity has been observed to sharplydrop immediately. Conductivity changes of 100 to 1,000 times have beenmeasured between the fully expanded foam and a practical degree of highcompression achievable with low pressures.

Charge build-up from ionization inside an open cell foam structure canpresent a problem. However, one solution is to utilize a liquid fillingof the foam material, as further discussed below.

A much larger and more uniform mechanical contact area of a foraminousroller surface with any paper between it and a photoreceptor providesgreatly improved mechanical tacking of the paper to the photoreceptor.Thus, the chances of premature toner transfer across any significant airgap between the paper and the photoreceptor are greatly reduced, sincethe paper is already mechanically held against the photoreceptor beforeit can be subjected to fields sufficient for toner transfer, either fromthe transfer roll or from charges deposited on the paper from pre-nipcorona. This is even more true of the fully insulative foraminousmaterial mode previously described. Uniform and positivepaper/photoreceptor contact, especially at the leading and trailingedges, is, of course, one of the principal advantages of a bias rolltransfer system as opposed to a corotron transfer system and aforaminous roll is superior in this regard.

Considering the liquid filled mode, all or part of the open cells may befilled with a suitable liquid. In addition to changing the electricalproperties the liquid filling can change the mechanical tackingproperties since the weight of the liquid and its hydrostaticcharacteristics causes it to apply a uniform pressure over a largecontact area between the outer insulating skin of the roller and thesupportsurface. Electrically, in addition to the reduction ofresistivity changes due to humidity, the liquid filling of theforaminous material has been observed to provide more uniform and higherdensity images.

With a liquid filled foam material the material does not have to be ascritically compressed for good transfer, rather the volume resistivityof the liquid is the controlling factor. This can be advantageousbecause it does not require as large a compression to achieve itseffect. A severe roller deformation is less desirable because it maypresent problems in permanent distortion of the roller or roller surfacespeed synchronization with the support surface. However, the same opencell material properties are utilized here to provide sufficient nipcompression to take advantage of the volume resistivity of the fluid andtherefore to provide a substantially lower resistance in the nip region.The substantial nip compression effectively prevents the use of such aroller as a liquid loaded development material applicator, which is notdesired in any case. The seal coating prevents intermixture of theinternal ionization control liquid with any developer material such asP.E.P. liquids.

Several different liquid materials have been utilized for the filling ofthe foraminous material. It will be appreciated that these noted hereare merely exemplary. Examples are silicone oils doped, for example,with tin salts to a selected conductivity. Several drops of Bis-Tri-N-Butyl Tin Maleat doping has been utilized. However, a mixture of65 percent GANEX and 35 percent Butyl Stearate has been found to be morereliable. Another example is a Sohio mix consisting of 3440 Sohio andisopropyl alcohol.

In conclusion, it may be seen that there has been described herein aforaminous roll system providing greatly improved operating propertiesand capable of overcoming many problems in electrostatographic systems.It will be obvious that the disclosed system is applicable to many otherelectrostatographic systems than those specifically discussed above. Oneexample is in TESI systems wherein latent electrostatic image chargepatterns are transferred from one support surface to another and thesurfaces are subsequently separated. The rollers of the invention canovercome some of the similar ionization control problems in such TESIsystems. Another possible application area would be induction chargingsystems in which a photoconductor on a transparent substrate is imageexposed in a temporarily high field region generated in the bias rollernip.

The exemplary embodiments described herein are presently considered tothe preferred; however, it is contemplated that further variations andmodifications within the purview of those skilled in the art can be madeherein. The following claims are intended to cover all such variationsand modifications as fall within the true spirit and scope of theinvention.

What is claimed is:

1. In electrostatographic apparatus wherein a transfer bias voltage isapplied between a roller electrode and a first support surface toprovide an electrical field for transfer therebetween while relativemovement is provided between said roller electrode and said firstsupport surface and said roller is variably deformed by said firstsupport surface at a roller nip area with prellll nip and post-nip areasat each side of said nip area, the improvement in said roller electrodecomprising:

an electrically conductive core to which said bias voltage is applied,spaced from said first support surface; and a thick highly compressibleroller body of foraminous open cell material extending between saidconductive core and said first support surface and having a substantialnormal uncompressed thickness;

said foraminous material occupying the space between said conductivecore and said first support surface with a multiplicity of smalldiscontinuities provided by said cells in said material and providing anionization control barrier;

said foraminous material being highly compressed between said conductivecore and said first support surface in said nip area to a thickness atleast approximately one-half of said normal uncompressed thickness;

said foraminous material being much less compressed in said pre-nip andpost-nip areas than in said nip area, having a much greater thicknessand porosity between said conductive core and said first support surfacein said pre-nip and post-nip areas than in said nip area, and saidforaminous material lying over a substantial area of said first supportsurface, for ionization control in said prenip and post-nip areas.

2. The apparatus of claim 1 wherein said foraminous material issubstantially fully compressed in the center of said nip area and saidconductive core closely approaches said first support surface in saidnip area.

3. The apparatus of claim 1 wherein said first support surface carrieselectrostatically attractable image development material in a liquid,and wherein said roller electrode is a blocking electrode forelectrophoretic imaging development thereof.

4. The apparatus of claim 1 wherein a second support surface member isengaged between said first support surface and said roller electrode,and said roller electrode is a bias transfer roller for electrostaticimage transfer of developer material from said first support surface tosaid second support surface.

5. The apparatus of claim I wherein the exterior of said foraminousroller body has a substantially liquid impervious seal coating which isresiliently conformable to said compression of said foraminous material.

6. The apparatus of claim 1 wherein said foraminous roller body isnon-conductive to said bias voltage and electrically insulates saidconductive core from said first support surface and does not affect theelectrical field therebetween.

7. The apparatus of claim 1 wherein said conductive core and saidforaminous roller body are curvalinear and said core has a radiusgreatly smaller than the normal uncompressed radius of said foraminousroller body.

8. The apparatus of claim 1 wherein said open cells of said foraminousroller body are air filled.

9. The apparatus of claim 1 wherein said open cells of said foraminousroller body are filled with a resistive liquid material.

10. The apparatus of claim 9 wherein said liquid material iselectrically conductive for said transfer bias voltage.

11. The apparatus of claim 1 wherein said foraminous material in saidnip is compressed sufficiently to collapse a substantial portion of saidopen cells in said nip.

12. The apparatus of claim 1 wherein said conductive core and saidroller body are cylindrical and coaxially mounted with said conductivecore uniformly wrapped with said foraminous material, and wherein theradius of said conductive core is less than approximately onehalf thenormal radius of said foraminous material.

13. The apparatus of claim 11 wherein said foraminous material iscompressed in said nip to approximately 20 percent of its normaluncompressed thickness.

14. The apparatus of claim ll wherein said foraminous material is aninsulator and is non-conductive to said transfer bias.

15. In electrostatographic apparatus wherein a transfer bias voltage isapplied between a roller electrode and a first support surface toprovide an electrical field for transfer therebetween while relativemovement is provided between said roller electrode and said firstsupport surface and said roller is variably deformed by said firstsupport surface at a roller nip area with prenip and post-nip areas ateach side of said nip area, the improvement in said roller electrodecomprising:

an electrically conductive core to which said bias voltage is applied,spaced from said first support surface; and

a thick highly compressible roller body of foraminous open cell materialextending between said conductive core and said first support surfaceand having a substantial normal uncompressed thickness;

said foraminous material occupying the space between said conductivecore and said first support surface with a multiplicity of smalldiscontinuities provided by said cells in said material and providing anionization control barrier;

said foraminous material being highly compressed between said conductivecore and said first support surface in said nip area to a thicknessgreatly less than said normal uncompressed thickness;

said foraminous material being much less compressed in said pre-nip andpost-nip areas than in said nip area and having a much greater thicknessand porosity between said conductive core and said first support surfacetherein over a substantial area of said surface for ionization controlin said pre-nip and post-nip areas;

wherein driving torque means apply torque to said roller electrode,resisted by said first support surface, for eccentrically distorting theconfiguration of said foraminous material so that said foraminousmaterial extends substantially further into said prenip region than saidpost-nip region over said first support surface, thereby providing agreater said corona control barrier in said pre-nip region then saidpost-nip region.

16. In electrostatographic apparatus wherein a transfer bias voltage isapplied between a roller electrode and a first support surface toprovide an electrical field for transfer therebetween while relativemovement is provided between said roller electrode and said firstsupport surface and said roller is variably deformed by said firstsupport surface at a roller nip area with prenip and post-nip areas ateach side of said nip area, the improvement in said roller electrodecomprising:

an electrically conductive core to which said bias voltage is applied,spaced from said first support surface; and a thick highly compressibleroller body of foraminous open cell material extending between saidconductive core and said first support surface and having a substantialnormal uncompressed thickness;

said foraminous material occupying the space between said conductivecore and said first support surface with a multiplicity of smalldiscontinuites provided by said cells in said material and providing anionization control barrier;

said foraminous material being highly compressed between said conductivecore and said first support surface in said nip area to a thicknessgreatly less than said normal uncompressed thickness;

said foraminous material being much less compressed in said pre-nip andpost nip areas than in said nip area and having a much greater thicknessand porosity between said conductive core and said first support surfacetherein over a substantial area of said surface for ionization controlin said pre-nip and post-nip areas;

wherein said foraminous material is electrically resistive and conductssaid transfer bias, and wherein the resistivity of said materialcompressed in said nip is rendered substantially lower than itsresistivity in said normal thickness by a substantial percentage ofcompressively collapsed cells in said nip, to provide increased niptransfer field strength.

17. The apparatus of claim 16 wherein foraminous material in said nip iscompressed to at least approximately one-half of said normaluncompressed thickness.

18. In electrostatographic apparatus wherein a transfer bias voltage isapplied between a roller electrode and a first support surface toprovide an electrical field for transfer therebetween while relativemovement is provided between said roller electrode and said firstsupport surface and said roller is variably deformed by said firstsupport surface at a roller nip area with prenip and post-nip areas ateach side of said nip area, the improvement in said roller electrodecomprising:

an electrically conductive core to which said bias voltage is applied,spaced from said first support surface; and a thick highly compressibleroller body of foraminous open cell material filled with an electricallyresistive liquid material extending between said conductive core andsaid first support surface and having a substantial normal uncompressedthickness; said foraminous material occupying the space between saidconductive core and said first support surface with a multiplicity ofsmall discontinuities provided by said cells in said material andproviding an ionization control barrier; said foraminous material beingcompressed between said conductive core and said first support surfacein said nip area to a thickness less than said normal uncompressedthickness; said foraminous material being much less compressed in saidpre-nip and post-nip areas than in said nip area and having a muchgreater thickness between said conductive core and said first supportsurface therein over a substantial area of said surface for ionizationcontrol in said pre-nip and postnip areas; wherein said foraminousmaterial with said resistive liquid material therein conducts saidtransfer bias, and wherein its resistivity in said nip is substantiallylower than its resistivity in said normal uncompressed thickness, toprovide increased nip transfer field strength; and wherein the exteriorof said foraminous roller body has a substantially liquid imperviousseal coating which is resiliently conformable to said compression ofsaid foraminous material.

1. In electrostatographic apparatus wherein a transfer bias voltage isapplied between a roller electrode and a first support surface toprovide an electrical field for transfer therebetween while relativemovement is provided between said roller electrode and said firstsupport surface and said roller is variably deformed by said firstsupport surface at a roller nip area with pre-nip and post-nip areas ateach side of said nip area, the improvement in said roller electrodecomprising: an electrically conductive core to which said bias voltageis applied, spaced from said first support surface; and a thick highlycompressible roller body of foraminous open cell material extendingbetween said conductive core and said first support surface and having asubstantial normal uncompressed thickness; said foraminous materialoccupying the space between said conductive core and said first supportsurface with a multiplicity of small discontinuities provided by saidcells in said material and providing an ionization control barrier; saidforaminous material being highly compressed between said conductive coreand said first support surface in said nip area to a thickness at leastapproximately one-half of said normal uncompressed thickness; saidforaminous material being much less compressed in said prenip andpost-nip areas than in said nip area, having a much greater thicknessand porosity between said conductive core and said first support surfacein said pre-nip and post-nip areas than in said nip area, and saidforaminous material lying over a substantial area of said first supportsurface, for ionization control in said pre-nip and post-nip areas. 2.The apparatus of claim 1 wherein said foraminous material issubstantially fully compressed in the center of said nip area and saidconductive core closely approaches said first support surface in saidnip area.
 3. The apparatus of claim 1 wherein said first support surfacecarries electrostatically attractable image development material in aliquid, and wherein said roller electrode is a blocking electrode forelectrophoretic imaging development thereof.
 4. The apparatus of claim 1wherein a second support surface member is engaged between said firstsupport surface and said roller electrode, and said roller electrode isa bias transfer roller for electrostatic image transfer of developermaterial from said first support surface to said second support surface.5. The apparatus of claim 1 wherein the exterior of said foraminousroller body has a substantially liquid impervious seal coating which isresiliently conformable to said compression of said foraminous material.6. The apparatus of claim 1 wherein said foraminous roller body isnon-conductive to said bias voltage and electrically insulates saidconductive core from said first support surface and does not affect theelectrical field therebetween.
 7. The apparatus of claim 1 wherein saidconductive core and said foraminous roller body are curvalinear and saidcore has a radius greatly smaller than the normal uncompressed radius ofsaid foraminous roller body.
 8. The apparatus of claim 1 wherein saidopen cells of said foraminous roller body are air filled.
 9. Theapparatus of claim 1 wherein said open cells of said foraminous rollerbody are filled with a resistive liquid material.
 10. The apparatus ofclaim 9 wherein said liquid material is electrically conductive for saidtransfer bias voltage.
 11. The apparatus of claim 1 wherein saidforaminous material in said nip is compressed sufficiently to collapse asubstantial portion of said open cells in said nip.
 12. The apparatus ofclaim 1 wherein said conductive core and said roller body arecylindrical and coaxially mounted with said conductive core uniformlywrapped with said foraminous material, and wherein the radius of saidconductive core is less than approximately one-half the normal radius ofsaid foraminous material.
 13. The apparatus of claim 1 wherein saidforaminous material is compressed in said nip to approximately 20percent of its normal uncompressed thickness.
 14. The apparatus of claim1 wherein said foraminous material is an insulator and is non-conductiveto said transfer bias.
 15. In electrostatographic apparatus wherein atransfer bias voltage is applied between a roller electrode and a firstsupport surface to provide an electrical field for transfer therebetweenwhile relative movement is provided between said roller electrode andsaid first support surface and said roller is variably deformed by saidfirst support surface at a roller nip area with pre-nip and post-nipareas at each side of said nip area, the improvement in said rollerelectrode comprising: an electrically conductive core to which said biasvoltage is applied, spaced from said first support surface; and a thickhighly compressible roller body of foraminous open cell materialextending between said conductive core and said first support surfaceand having a substantial normal uncompressed thickness; said foraminousmaterial occupying the space between said conductive core and said firstsupport surface with a multiplicity of small discontinuities provided bysaid cells in said material and providing an ionization control barrier;said foraminous material being highly compressed between said conductivecore and said first support surface in said nip area to a thicknessgreatly less than said normal uncompressed thickness; said foraminousmaterial being much less compressed in said pre-nip and post-nip areasthan in said nip area and having a much greater thickness and porositybetween said conductive core and said first support surface therein overa substantial area of said surface for ionization control in saidpre-nip and post-nip areas; wherein driving torque means apply torque tosaid roller electrode, resisted by said first support surface, foreccentrically distorting the configuration of said foraminous materialso that said foraminous material extends substantially further into saidpre-nip region than said post-nip region over said first supportsurface, thereby providing a greater said corona control barrier in saidpre-nip region then said post-nip region.
 16. In electrostatographicapparatus wherein a transfer bias voltage is applied between a rollerelectrode and a first support surface to provide an electrical field fortransfer therebetween while relative movement is provided between saidroller electrode and said first support surface and said roller isvariably deformed by said first support surface at a roller nip areawith pre-nip and post-nip areas at each side of said nip area, theimprovement in said roller electrode comprising: an electricallyconductive core to which said bias voltage is applied, spaced from saidfirst support surface; and a thick highly compressible roller body offoraminous open cell material extending between said conductive core andsaid first support surface and having a substantial normal uncompressedthickness; said foraminous material occupying the space between saidconductive core and said first support surface with a multiplicity ofsmall discontinuites provided by said cells in said material andproviding an ionization control barrier; said foraminous material beinghighly compressed between said conductive core and said first supportsurface in said nip area to a thickness greatly less than said normaluncompressed thickness; said foraminous material being much lesscompressed in said pre-nip and post nip areas than in said nip area andhaving a much greater thicKness and porosity between said conductivecore and said first support surface therein over a substantial area ofsaid surface for ionization control in said pre-nip and post-nip areas;wherein said foraminous material is electrically resistive and conductssaid transfer bias, and wherein the resistivity of said materialcompressed in said nip is rendered substantially lower than itsresistivity in said normal thickness by a substantial percentage ofcompressively collapsed cells in said nip, to provide increased niptransfer field strength.
 17. The apparatus of claim 16 whereinforaminous material in said nip is compressed to at least approximatelyone-half of said normal uncompressed thickness.
 18. Inelectrostatographic apparatus wherein a transfer bias voltage is appliedbetween a roller electrode and a first support surface to provide anelectrical field for transfer therebetween while relative movement isprovided between said roller electrode and said first support surfaceand said roller is variably deformed by said first support surface at aroller nip area with pre-nip and post-nip areas at each side of said niparea, the improvement in said roller electrode comprising: anelectrically conductive core to which said bias voltage is applied,spaced from said first support surface; and a thick highly compressibleroller body of foraminous open cell material filled with an electricallyresistive liquid material extending between said conductive core andsaid first support surface and having a substantial normal uncompressedthickness; said foraminous material occupying the space between saidconductive core and said first support surface with a multiplicity ofsmall discontinuities provided by said cells in said material andproviding an ionization control barrier; said foraminous material beingcompressed between said conductive core and said first support surfacein said nip area to a thickness less than said normal uncompressedthickness; said foraminous material being much less compressed in saidpre-nip and post-nip areas than in said nip area and having a muchgreater thickness between said conductive core and said first supportsurface therein over a substantial area of said surface for ionizationcontrol in said pre-nip and post-nip areas; wherein said foraminousmaterial with said resistive liquid material therein conducts saidtransfer bias, and wherein its resistivity in said nip is substantiallylower than its resistivity in said normal uncompressed thickness, toprovide increased nip transfer field strength; and wherein the exteriorof said foraminous roller body has a substantially liquid imperviousseal coating which is resiliently conformable to said compression ofsaid foraminous material.